Set http_proxy and related env vars for containers

[dakale]

We dont pass the Runner's environment through to containers so http_proxy is not necessarily picked up

Work for the actionable parts of https://github.com/github/c2c-actions-runtime/issues/339

Added a variable to track the actual value of no_proxy to avoid having to rebuild it from the parts we split it into

Simple canary workflow:

jobs:
  proxy-host:
    runs-on: [ self-hosted, linux ]
    steps:
    - run: |
        printenv
        curl -v httpbin.org/uuid
  proxy-container:
    runs-on: [ self-hosted, linux ]
    container:
      image: ubuntu
    services:
      squid:
        image: dakale/squid
    steps:
    - run: |
        apt-get -o Acquire::http::proxy=false update > /dev/null && apt-get -o Acquire::http::proxy=false install -y curl > /dev/null
        printenv
        curl -v httpbin.org/uuid

Traffic through squid is shown by cache headers added by the proxy